there's not a single issue that you have a defense for Biden that isn't "Yeah he's bad but Trump will do it worse."

Let's say this is accurate. Forget the actual good things he's done which others have pointed out already, let's say he's literally going to do nothing good, and the only thing he has on Trump is that he's not as bad.

It's still a moral imperative to vote for him. You have a choice between some people suffering, and more people suffering. Choosing to let more people suffer so that you can pretend you had nothing to do with it makes you partially culpable. It's like the trolley problem, but the choice is between 1 person or that 1 person and 5 more. The only thing you can do is pull the lever and make the trolley miss the 5 people, and you're complaining that one person will still die, so you'd rather see all 6 dead than participate the system and save anyone.

You can pretend that it's a way of protesting all you want, you can pretend that maybe it'll signal that the system needs to change, but not engaging with the system out of protest is visually identical to not engaging with the system because you don't care at all. Vote in primaries, spend the other 1,460 days between elections protesting all you want, but on election day, you have exactly two choices: some people die, or those same people and more die. Choosing not to vote is only telling the powers that be that you don't care which one happens.